The long-term goal of the research in our laboratory is to understand the molecular mechanisms of vascular endothelial cell (EC) and smooth muscle cell (SMC) growth control in vivo. These two cell types are generally in a quiescent growth state; however, the onset of EC proliferation is associated with the pathogenesis of numerous "angiogenic diseases" (e.g., solid tumor growth) while SMC accumulation occurs during the development of atherosclerosis and restenosis after vessel wall injury. We have used a gene discovery approach to study polypeptide growth factor-stimulated cell proliferation. Several novel growth factor-inducible genes have been identified, and one of these genes, named Fn14, is the subject of this competitive renewal application. Recent studies have indicated that the Fn14 plasma membrane protein is a receptor for the tumor necrosis factor (TNF)-related cytokine named TWEAK. This cytokine is a type II transmembrane protein that occurs in both cell-associated and soluble forms. Although TWEAK has been reported to be a weak inducer of cellular apoptosis, it is thought to primarily function as a survival and growth factor. Indeed, it was reported in 1999 that TWEAK is an EC mitogen and an angiogenic factor in the rat cornea angiogenesis assay. Additional studies are required to further evaluate the biological functions of TWEAK and Fn14 using both cell culture- and animal-based experimental systems. Accordingly, the Specific Aims of this proposal are: 1) to confirm and extend previous results indicating that TWEAK can stimulate EC proliferation in vitro and promote angiogenesis in vivo. In addition, we will determine whether TWEAK is a chemotactic factor for EC and if TWEAK biological activity on EC is mediated via binding to the Fn14 receptor; 2) To determine whether endogenously- expressed TWEAK can function in an autocrine manner via the Fn14 receptor to regulate EC survival/proliferation in vitro; 3) To begin investigating the TWEAK signal transduction pathway by identifying proteins that bind the Fn14 cytoplasmic tail and by determining whether TWEAK addition to quiescent EC or Fn14 overexpression in transfected HEK293 cells activates signaling pathways that promote cell survival/proliferation; and 4) To determine the phenotypic consequences of Fn14 deficiency in vivo. It is anticipated that these studies will provide important information on the functions of the TWEAK and Fn14 proteins and their role in vascular cell biology.